This invention relates to two-part moisture curable coating compositions capable of forming polyurethane-polysiloxane-Si (PU-PDMS-Si) organic-inorganic hybrid networks having improved mechanical strength and excellent foul releasing property. The coating compositions are useful in the field of antifouling coating.
Biofouling occurs everywhere in the marine environment and is a significant problem for marine artificial objects. One approach to “shed” fouling microorganisms is using self-cleaning foul releasing coatings based on silicone elastomers. Polydimethylsiloxane (PDMS) based silicone elastomers foul releasing coatings have rubbery elasticity, very low surface energy and smooth surface. On these surfaces the adhesion strength of marine organisms is low and they detach from the coating surface under shear stress generated by hydrodynamic drag. However, PDMS is extremely soft, and does not have good damage tolerance. Since the silicone component easily wears off, the silicone rubber based antifouling coating requires frequent reapplications, which is very fussy, costly and time consuming.
One effective approach to improve the mechanical properties of PDMS based silicone coating is to blend PDMS with other polymers with better mechanical properties, such as polyurethane (PU). Polysiloxanes and polyurethanes possess very different, but highly useful, physical and mechanical properties which have led to their widespread use in countless applications. Polyurethanes stand out by virtues of mechanical strength, elasticity, adhesion resistance and abrasion resistance in the combination with polydimethylsiloxane in foul releasing coatings. However uniform physical blends of polysiloxanes and polyurethanes are very hard to be obtained, due to the highly incompatible properties of these resins and their pronounced tendency to undergo phase separation following their initial admixture. Silane terminated PU are already known in sealant, adhesive or binders' area. U.S. Pat. Appl. 2007/0129528 A1 describes a two-package system of polyurethane-polysiloxane resin mixture, where the first part contains a moisture-curable silylated polyurethane resin and a crosslinker for silanol-terminated diorganopolysiloxane, the second part contains silanol-terminated diorganopolysiloxane and a condensation catalyst. The resulting mixture exhibits uniform physical appearance, high elastic modulus, high tensile strength and high thermostability. However, the polyols disclosed to be utilized for the preparation of the silane terminated PU are polyether polyols, polyester polyols, polyetherester polyols and polyesterether polyols. In this system, cross-linked products of silylated PU prepolymer and silanol terminated polysiloxane have a variety of advantageous properties, but still have disadvantages especially in environmental durability over time, such as unsatisfactory UV resistance, oxidation resistance and alkali resistance. There is a desire to overcome these disadvantages and seek more cost effective and environmental benign raw materials in manufacture of the PU-PDMS-Si hybrid systems which are potentially used as foul releasing coatings.
The inventors surprisingly found that natural oils, an abundant renewable resource, can be successfully employed in the production of PU through the introduction of hydroxyl functional groups into the products which exhibit excellent mechanical properties such as compressive strength and elasticity and also excellent environmental durability. Moreover, the hydrophobic nature of the polymers produced from natural oil polyol (NOP) and their derivatives further provided excellent physical and chemical properties to the PU-PDMS-Si system. Novel moisture curable coating compositions derived from the PU-PDMS-Si system based on NOPs were found to have not only excellent mechanical properties, low surface energy and excellent foul-release properties, but also improved environmental durability performances such as UV resistance, oxidation resistance and alkali resistance. These advantageous properties enable the PU-PDMS-Si system to be extremely suitable for the application in the field of foul releasing coatings. The coating showed special surface morphology due to the phase separation of silylated PU and silanol PDMS. In this coating system, microphase separation occurring at the surface of the coating results in special surface structure. Domain size can be controlled by properly select silylated PU and silanol terminated polysiloxane with proper type and molecular weight. The coatings have well-defined surface structure and have been demonstrated to inhibit settlement of fouling organisms.